Roberts had recently quit his job and sold his belongings, saying he was moving to Hawaii.

Photo: Handout, Getty Images

Roberts had recently quit his job and sold his belongings, saying...

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Flags are at half staff and a sign is a reminder of Christmas Wednesday Dec. 12, 2012, at the entrance to the scene of a multiple shooting Tuesday at Clackamas Town Center Mall in Portland, Ore. Jacob Tyler Roberts killed two people and himself at the mall. (AP Photo/Greg Wahl-Stephens)

Photo: Greg Wahl-Stephens, Associated Press

Flags are at half staff and a sign is a reminder of Christmas...

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Two white roses lie at the edge of the almost empty parking lot Wednesday Dec. 12, 2012, outside the scene of a multiple shooting Tuesday at Clackamas Town Center Mall in Clackamas, Ore. A gunman who opened fire on shoppers at a Portland mall had no connection to the two people he fatally shot and wanted to kill as many people as possible, police said Wednesday. (AP Photo/Greg Wahl-Stephens)

Photo: Greg Wahl-Stephens, Associated Press

Two white roses lie at the edge of the almost empty parking lot...

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Kyle Price, left, and Angela Price who live directly across from the entrance to the scene of a multiple shooting Tuesday at Clackamas Town Center Mall, take a video to send to friends as they stand across the street in Portland, Ore., Wednesday Dec. 12, 2012. (AP Photo/Greg Wahl-Stephens)

Photo: Greg Wahl-Stephens, Associated Press

Kyle Price, left, and Angela Price who live directly across from...

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Clackamas County Sherriff Craig Roberts speaks at a press conference Wednesday Dec. 12, 2012, about the multiple shooting Tuesday at Clackamas Town Center Mall in Portland, Ore. Jacob Tyler Roberts killed two people and himself in the shooting rampage. (AP Photo/Greg Wahl-Stephens)

Photo: Greg Wahl-Stephens, Associated Press

Clackamas County Sherriff Craig Roberts speaks at a press...

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Flags fly at half staff Wednesday Dec. 12, 2012, at the entrance to the scene of a multiple shooting Tuesday at Clackamas Town Center Mall in Clackamas, Ore. Jacob Tyler Roberts killed two people and himself in the shooting rampage. (AP Photo/Greg Wahl-Stephens)

Leslie King, left, and Tenille Beseda carry flowers Wednesday Dec. 12, 2012, to place at the entrance to the scene of a multiple shooting Tuesday at Clackamas Town Center Mall in Portland, Ore. A gunman who opened fire on shoppers at a Portland mall had no connection to the two people he fatally shot and wanted to kill as many people as possible, police said Wednesday. (AP Photo/Greg Wahl-Stephens)

Photo: Greg Wahl-Stephens, Associated Press

Leslie King, left, and Tenille Beseda carry flowers Wednesday Dec....

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Clackamas County Sherriff Craig Roberts explains a diagram at a press conference Wednesday Dec. 12, 2012, about the multiple shooting Tuesday at Clackamas Town Center Mall in Portland, Ore. Jacob Tyler Roberts killed two people and himself in the shooting rampage. (AP Photo/Greg Wahl-Stephens)

Photo: Greg Wahl-Stephens, Associated Press

Clackamas County Sherriff Craig Roberts explains a diagram at a...

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Mall shoppers and employees call after being released from the Clackamas Town Center mall Tuesday Dec. 11, 2012. Two people were killed and at least one other was shot at the Clackamas Town Center in Portland Oregon by a man brandishing a semiautomatic rifle, according to police reports. Emergency dispatchers received reports of a shooting near Macy's and that a man was seen with an AR-15 rifle near the mall's food court.(AP Photo/Thomas Boyd, The Oregonian)

Portland, Ore. -- Gunfire rang out in the mall food court, instantly transforming a casual afternoon of holiday shopping into a nightmare. The shooter, armed with a rifle, was dressed in dark clothing and wore a hockey-style face mask.

As panicked shoppers fled for cover, workers ushered some into hiding places within stores, or helped them to the exits. The first officers to arrive formed groups and rushed into the chaos, rather than waiting for the more heavily armed SWAT team.

The quick mobilization of mall workers and police reflects the reality that, while mass shootings are rare, they have forced authorities to rehearse for such outbreaks of violence as if they are the norm.

"This could have been much, much worse," Clackamas County Sheriff Craig Roberts said.

Roughly 10,000 people were inside the Clackamas Town Center on Tuesday afternoon, when police say Jacob Tyler Roberts, 22, armed himself with an AR-15 semiautomatic rifle he stole from someone he knew, and went on a rampage that left two people dead.

The sheriff said the rifle jammed during the attack, but the shooter managed to get it working again. He later shot himself. The sheriff and Roberts are not related.

As authorities tried to determine a motive for a shooting they said had no specific targets, details emerged about Roberts from acquaintances and neighbors. They described him as relaxed, friendly and outgoing.

"Jake was never the violent type," Roberts' ex-girlfriend, Hannah Patricia Sansburn, told ABC News. "His main goal was to make you laugh, smile, make you feel comfortable. You can't reconcile the differences. I hate him for what he did, but I can't hate the person I knew because it was nothing like the person who would go into a mall and go on a rampage."

Sansburn said Roberts had recently quit his job at a gyro shop in Portland and sold all of his belongings, telling her that he was moving to Hawaii. He was supposed to take a flight Saturday but told her he got drunk and missed it.

"And then this happens. ... It makes me think, was he even planning on going to Hawaii?" Sansburn said.

A former neighbor of Roberts said that he liked to play video games and never seemed troubled.